Broadly speaking, there are two kinds of magnets, namely, permanent magnets and electromagnets. Magnets of the former type are made of a material which is permanently magnetized. Such magnets are devoid of windings and require no external source of power to produce a magnetic field. And neither can they be easily "turned off."
On the other hand, electromagnets have a ferrous core wound with a coil of wire through which electric current is passed for creating a magnetic field. The magnetic field persists so long as electric current persists; clearly, electromagnets may easily be turned on and off by controlling the electrical power to which the coil of wire is connected. The invention involves a magnet of the latter type and, particularly, involves an alternating current (AC) electromagnet with a moving armature structure, both as used with an AC electric motor.
Electric motors are often used to drive conveyors, door openers, cranes and the like. Often, the driven machine requires rapid stopping and load-holding when no electrical power is being applied to the motor. The seemingly-inconsistent requirements of driving power on the one hand and rapid load-stopping and good load-holding on the other are met by an electric motor which has a spring-set, electromagnetically-released disc brake mounted with the motor, usually in a common housing.
A disc brake of this type has one or more "pancake-like" friction discs, each with a center opening splined or otherwise attached to the motor shaft and rotating with such shaft. Stationary ring-like discs are keyed to the housing to prevent such discs from rotating and are alternately stacked with the friction discs.
When the electromagnet is de-energized, springs urge the friction discs (which are splined to and rotate with the motor shaft) and ring-like discs (which are prevented from rotating) against one another. The motor is thereby brought to a rapid stop. Further, the motor and the "load" attached to it, e.g., a conveyor or the like, are held in the stopped position. When the motor is again energized, the brake electromagnet is also energized and compresses the brake-applying springs to release the brake and permit the motor to rotate freely. A foremost manufacturer of magnetic disc brakes used with electric motors is Dings Company of Milwaukee, Wis.
Motors combined with electromagnetic brakes are depicted in a number of prior art patents and U.S. Pat. Nos. 4,022,301 (Hansen); 4,7698,269 (Lindner et al.) and 5,186,287 (Lindner et al.) are exemplary. The electromagnet arrangement depicted in the Lindner et al. patents use what is often referred to as a "T" type electromagnet. Electromagnets of such type are so named because the armature, that moving portion drawn toward and into contact with the electromagnet when such electromagnet is energized, is shaped like the letter "T."
Another common type is known as an "E" type and has an electromagnet shaped like the letter "E." The armature is a rectangular block and its flat face is drawn toward and into contact with the flat end faces of the three "legs" of the electromagnet when the latter is energized. The invention involves an electromagnet of the "E" type.
In a known electromagnetic brake of the "E" type, the armature is rigidly attached to and forms part of an armature plate assembly. Such assembly includes a substantially flat plate with the armature block mounted near one end. As the electromagnet is energized and deenergized, the armature plate assembly pivots very slightly about the end opposite the armature block as the armature block is alternately attracted to and released from the "E" magnet.
When new, the armature block is adjusted (or should be adjusted) so that the magnet-contacting surface of such block is parallel to the faces of the electromagnet. If that is not done, there is an audible AC "hum" when the magnet is energized since the surface of the armature block contacting the faces of the magnet legs is not parallel to such faces. In a more extreme case, such sound can be quite annoying. And since the "E" magnet is rigidly mounted on support studs or the like, there is no opportunity for self-alignment of the faces of the "E" magnet legs to the armature block.
One approach to solving the noise problem involves a "floating" mount arrangement. The armature block is loosely attached to the armature assembly plate by bolts through large-clearance holes in the plate and through holes in a resilient pad interposed between the block and the plate. The block is quite free to tilt with respect to the plate and, it is reasoned, free to align itself parallel with the faces of the "E" magnet legs.
However, it was found that initial contact of the armature block with such faces was in the nature of a "line" contact rather than an "area" contact. As a result, the block face was "Brinnelled" (hardened by impact) and the assembly was noise-free for only a few hundred thousand cycles of brake operation.
And there are other problems with known electromagnetic brake magnet structures. Over an extended time, such structures endure a good deal of "hammering" (as the magnet is repeatedly energized and de-energized) and bolts holding the armature block to the armature plate often loosen. Known efforts to prevent bolt loosening include applying Loc-Tite.RTM. holding compound but even this has not been uniformly successful. Some have resorted to welding to prevent the armature block from loosening.
Yet another problem with known electromagnetic brake magnet structures is that they are relatively expensive to build. Of course, reductions in the cost of such structures would help the manufacturer realize a greater return on investment and/or provide the purchaser with an incrementally lower cost unit.
A structure which significantly reduces AC "hum" in an electromagnetic brake over an extended operating life, which securely retains armature block mounting bolts and which is less expensive to manufacture would be an important advance in the art.